(1) Field of the Invention
This invention concerns piezoelectric/pyroelectric elements, and relates in particular to a novel construction of such an element, together with a process for its production.
(2) Description of the Prior Art
A piezoelectric material is one that, in the form of an article of an appropriate physical shape, produces across itself an electric voltage when subjected to mechanical strain (and mechanically deforms itself when subjected to an electric field); a pyroelectric material is one where heating produces the electric voltage.
In each case the electric voltage is conventionally picked up, or the electric field conventionally applied, via a pair of electrodes mounted upon the article's two relevant faces. The original piezoelectric materials were all naturally-occurring anistropic crystals (for example, of quartz or Rochelle salt), but recently it has been discovered that certain plastics substances can be made into films or sheets having a "crystalline" form which can then be treated so as to become very strongly piezoelectric and/or pyroelectric. One particular such plastics substance is poly(vinyl difluoride)--also know as polyvinylidene fluoride, and referred to as PVF.sub.2 or PVDF; thin films of partially .beta.-phase PVDF can be made very strongly piezo- or pyroelectric by a process in which they are heated to near the "softening" temperature, placed in a strong polarising electric field, and then (while in the field) allowed to cool slowly back to room temperature. It is not yet fully understood exactly how this treatment causes the observed effects, but it is surmised that, at least in part, the polarising field causes the soft PVDF's electrically-asymetric molecules to re-align, that the state of alignment is retained when the PVDF cools and "sets", and that subsequent mechanical strain or heating causes a temporary distortion of the aligned molecules and a corresponding charge separation which results in the observed generated electric voltage.
Piezoelectric and pyroelectric elements made from polarised plastics substances such as PVDF are of considerable interest in a number of fields (a general discussion of polarised PVDF and its uses is in The Marconi Review, Fourth Quarter, 1976). Unfortunately, they also suffer from a number of not insignificant drawbacks.
For a number of applications it can be shown that the sensitivity of a fully-polarised plastics film piezoelectric element is in theory greater the thicker the film. Regrettably, however, the maximum thickness of fully-polarised plastics film obtainable is not sufficient--for many of these applications--to give the desired sensitivity. Full polarisation is achieved by applying the maximum possible polarising electric field; the limit of this field is that at which thermally-activated electrical breakdown occurs; and--as is typical for many insulator materials--the breakdown level decreases rapidly as the film thickness increases. The thickest fully-polarised plastics films currently available are about 50 microns thick--and this is simply not thick enough to give the desired sensitivity.
Rather surprisingly we have now found that a very satisfactory thick piezoelectric/pyroelectric element can be made from these plastics films if, instead of using a film which is itself thick, there is used a laminar stack of films which are themselves thin, the final element being formed by first constructing the stack from unpolarised films, and then polarising the whole stack in one, to make the desired element.